The invention makes use of coherent light, such as lasers and an interferometer, to study the displacements temporally or over time. Such techniques have been contemplated in the present applicants' co-pending UK Patent Application No. 0402914.6 and WO 04/003589, both of which are concerned with seismic exploration. The present invention is more generally applicable.
In the earlier cases, the techniques involved tracing fast interferometric signals closely, to calculate accurately the displacements of the object. High sampling frequencies were required, and the displacements were found after integration of a large number of separate sequential recordings. In the present application, the system is not tracing the fast interferometric signals, but actually tracing the slow signals in the system.
This simplifies the design of the system considerably, and requires less-expensive components.